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Bacteria using bacteria

Editor's Selection IconThere are lots of things I enjoy about studying bacteria. I love their biochemistry and the secret inner workings of their metabolic pathways. I love that everything they do the manage within the confines of a single cell, and I love that you can go in there with a wrench and hit some genes until they make what you want.

But what I'm really enjoying exploring at the moment is more ecological bacteriology; how bacteria interact with their environment. How they respond to changes to stresses and, most importantly, to other bacteria. In my last post I covered how natural throat bacteria can help destroy dangerous pathogens such as MRSA Staph aureus so today I'm going to look at almost the opposite; how some bacteria can give each other a helping hand in order to infect humans.

Campylobacter jejuni is a bacteria that I feel a special affinity for because I've worked with it, back in my first ever summer project. Unfortunately it's not a very nice bacteria and can lead to bad stomach illnesses with some rare but quite threatening complications. It's found in chicken meat and cheese as it is perfectly capible of surviving happily in animals without causing them any diseases.

One of the problems with working with Campylobacter jejuni (henseforth shortened to Campy which is what we called it in the lab) is that it's very fussy about the amount of oxygen it's in. Campy is microaerophilic, which means it needs oxygen, but only small amounts, give it too much and all the cels die on you. This problem was solved in the lab by using tightly sealed containers and special packs of ... stuff ... which were put inside the containers to create the right conditions. But this does raise an important question; if the bacteria is so difficult to culture on a plate in the lab then what the hell is it doing surviving on the surface of chicken meat!

A recent study (reference below) found what you've probably guessed if you were reading this post closely, the Campy were being aided by the surrounding bacteria. The picture below shows both Campy and a bacteria called Pseudomonas putida in close interaction, with long fibrelike structures connecting them. Noone seems to be really sure what the fibre-like structures are, they may be being used for chemical communication, or they may just be keeping the bacteria in close physical contact.

The campy is the more slender and slighly spiral shaped bacteria in the centre, the others are Putida. Image from the reference.

Both bacteria were identified as being in close contact, as well as being seen together under the microscope. Further experiments were done to show that the Putida was required for Campy survival - different Campy strains were grown in both the presence and absence of the supporting Putida to see how long they could survive in completely aerobic conditions. The results are kind of hilarious, without the help of the Putida bacteria the Campylobacter just die, really quickly (image from the reference):

Figure A (top) shows the Campy with Putida grown as well, Figure (B) shows the Campy grown alone. You don't really have to be particularly good at science to interpret that one! Interestingly it was found that the interaction between different strains of both Campy and Putida was fairly specific as well, as you can see in the graph above, only three of the Campy strains have survived past 50 hours with the help of this particular Putida. Three of the Campy's still die, although they surive longer than with no help at all.

As Putida are areobic, the most likely explanation for how they are helping is that they create a microaerophilic microenvironment within their immediate surroundings. This is the kind of environment that it is thought Campy will naturally migrate to. This might be less of a helping relationship and more of a seriously exploitative one, with the Campylobacter swarming as quickly as possible towards the environment created by the Putida and then wrapping them all up in a sticky mesh to stop them moving away.

This special relationship is not applicable for all Campys, in other environments such as in humans and in chicken poo the Campy exist fine on their own, but in the highly aerobic environment of the meat surface they rely on other bacteria to survive. The implications for treatment of bacteria are intreguing (especially for antibiotic resistant strains of Campy) but it is another reminder that despite laboratory conditions bacteria do not just exist in isolation. They inhabit a whole tiny world, with challenges of it's own, surrounded by other bacteria that change their envirnment both for better and for worse.

6 comments:

Faz
said...

It is incredible how many infections are the result of not just one simple interaction between pathogen and host, but a complex process modulated by the local bacterial community. I suspect that there are subtle interactions like this that play a role in many infectious diseases.

This is a legitimate question and I swear I'm not trying to be a grammar snob. I noticed you used bacteria as both singular and plural in your post. When writing about bacteria, do bacteria people not give much thought to the distinction between bacterium and bacteria? I know stuff like this happens frequently in science vernacular and I'm just curious if this is one of those cases.Great article by the way!

@Faz: Given the sheer numbers of bacteria both inside and outside the body it wouldn't surprise me at all! It's been really interesting reading about it lately.

@Casey: don't worry about asking questions. :) I think I just tend to use the plural word 'bacteria' by default (because there will rarely be just one of them) and only use 'bacterium' when I am specifically talking about only one. In normal speech (or in hastily typed out blog posts) the distinction can go somewhat out the window; the word 'bacterium' is rather rare though, so if it's used it's safe to assume there really is only one.

(As an example, the caption under the first picture really should have said 'bacterium' as it was referring to only one bacterium.

Wow, like Faz I wonder what else must be out there. How many bacteria might be unculturable, because they need the right partners, micro-environment or something else we don't know about? Not everyone likes marmite, so not every bacterium digs LB-Agar I suspect..

I sadly don't have access to the paper, but do you know if the authors really show if the Campylobacters exploited Putida? Did the Putida grow less quickly in pthe presence of Campylobacter, for example? Did the Campylobacter really actively migrate to the Putidas as you described?

@Lucas: There are millions of bacteria that don't grow in the lab, they are called 'unculturable' bacteria and you only know they are there through getting a sample of (say) soil and doing random sequencing on bits of it. All sorts of weird and wonderful things show up!

The paper went as far as showing the interaction, but did no further work to suggest it was in anyway a detrimental thing for the putida (although that would be interesting). You do see campy swarming towards them though, as they swarm towards any areas with the right oxygen levels.

In regards to unculturable bacteria I once read a great bio of an Australian researcher who specialised in recalcitrant bacteria. He would take a soil sample, dilute it several orders of magnitude then plate on a range of media from as minimal as is possible to rich media but also include minimal media with yeast extract or amoeba extract or nematode extract etc. I cant remember the number he had discovered but it was lotz! It was certainly an interesting research approach.